By CLAUDIO G. SEGRE; Claudio G. Segre is a professor of European history at the University of Texas at Austin.

Published: September 10, 1989

Attention, opera composers and librettists: this slight novel may provide wonderful material. Consider the premise of this last work by the Czechoslovak novelist, mathematician and classical scholar Leo Perutz (1882-1957): Leonardo - the Leonardo, of course - is having trouble completing ''The Last Supper.'' He can't find a suitable model for Judas, a contemporary face that will convey the mystery and anguish of the betrayal.

Enter Joachim Behaim, a proud German merchant who has sworn to recover a debt from a usurer named Boccetta. While conniving against Boccetta, Behaim falls hopelessly in love - with Boccetta's daughter Niccola. And in observing Behaim's decision to sacrifice love for vengeance, Leonardo finds the inspiration to complete his masterpiece.

Reminds you of Kafka? Hardly, though the publisher claims that Perutz has ''sometimes been compared'' to his brooding countryman. Rather, in Eric Mosbacher's smooth translation from the German, the charm of ''Leonardo's Judas'' lies in its color, its old-fashioned melodrama and morality. Real characters mix with imaginary ones on Perutz's historical sets. And those sets themselves seem made for opera: the ducal court in Milan; the rustic inn where Behaim carouses with artists and craftsmen; the wretched hovel that is Boccetta's home; the churches and inns where the lovers tryst; the hospital where the mysterious poet Mancino dies and the denouement takes place.

But although this scenery is beguiling, I had some trouble with the novel's ironies and its twists of plot. I don't quite believe, for example, that Mancino climbed in the window of Boccetta's house to return the money that Niccola, out of love for Behaim, stole from her father. Or that Behaim turned away from his true love so easily. Or that Niccola would really say she would never have loved Behaim ''if I had known he had Judas's face.'' But in an opera, I'd believe - and I might even cry a little.